In charged-particle-beam (CPB) projection microlithography as used in the fabrication of integrated circuits, a circuit pattern defined by a reticle or mask (these terms are used interchangeably herein) is irradiated with a charged particle beam to transfer the pattern defined by the reticle to a sensitized substrate (e.g., a semiconductor wafer). In recent years, CPB projection-microlithography apparatus ("pattern-transfer apparatus") have been developed that exhibit improved resolution of the transferred pattern and improved product throughput (i.e., the number of semiconductor wafers that can be exposed with a pattern per unit time). With certain conventional CPB pattern-transfer apparatus, one or more entire die patterns defined on a reticle are transferred onto the wafer in a single exposure. A "die" is a pattern coextensive with the bounds of an integrated circuit or other device to be transferred onto the wafer (usually multiple dies are exposed at respective locations on the wafer).
It is difficult to produce a reticle for a CPB pattern-transfer apparatus that transfers an entire die in a single exposure while also providing the high resolution and circuit densities demanded in recent years. In addition, conventional CPB pattern-transfer apparatus that transfer an entire die per exposure cannot satisfactorily control aberrations arising in the CPB optical system through which the charged particle beam passes, especially over a large optical field covering one or more dies. To solve this problem, CPB pattern-transfer apparatus have been proposed in which a pattern to be transferred is divided into multiple field segments (termed "mask subfields") that are individually and separately exposed. Such a pattern is typically transferred using a "step-and-repeat" transfer scheme in which the individual mask subfields are sequentially transferred to corresponding "transfer subfields" on a wafer or other sensitized substrate. The transfer subfields are produced on the wafer surface in locations relative to each other such that the transfer subfields are "stitched" together in the correct order and alignment to reproduce the entire die pattern on the wafer surface.
Various mechanical and environmental conditions can cause distortions in the mask (or reticle) from which the patterns are transferred. Such distortions include, but are not limited to, distortions that occur as a mask pattern is formed on the reticle, mechanical distortions of the reticle that occur when the reticle is mounted to a reticle stage or other reticle holder, and thermal distortions arising from changes in reticle temperature due to exposure of the reticle to a charged-particle beam. Whenever a mask pattern is transferred to a substrate using a distorted reticle, the projected mask pattern on the substrate is typically distorted. Such distortion decreases resolution of the transferred subfield images and the positional accuracy with which mask-subfield images are formed on the substrate. This results in poor accuracy with which the transfer subfields are "stitched" together.
Accordingly, there is a need to provide CPB projection-microlithography reticles, apparatus, and methods for correcting optical errors in pattern images projected from a distorted reticle before exposure of the substrate to the pattern defined by the reticle.